Light equalizer

ABSTRACT

A light equalizer having an output aperture across which each area element exhibits substantially the same emission intensity regardless of the distribution of light into the input aperture of the equalizer. The equalizer is embodied in a stack of a plurality of transmissive elements, a number of which provide a multiplicity of light scattering centers. The exterior surface of the equalizer is covered with an internally reflective coating except for the input and output apertures.

This application is a division of our copending application Ser. No.458,124 filed Apr. 5, 1974, now U.S. Pat. No. 3,887,262 for StackedEqualizers.

The present invention relates to optical devices and more particularlyto precision optics requiring a substantially uniform light source.

In order to achieve wavelength filtering in precision optical equipment,interference filters are often employed because the band-passcharacteristics of the latter can be quite precisely tailored. This isparticularly true in equipment, such as many types of analyticalspectrophotometers, operating in the infrared. However, thecharacteristics of an interference filter depend upon the thickness ofthe films forming the filter, and these characteristics will change asthe angle of incident light changes. This dependence of band-passcharacteristics on the angle of incident light is largely due to thechanges in the optical path length through each layer of film as theangle of incidence changes. Thus, for moderate changes of the angle fromthe normal, the effect is to shift the spectral characteristics of thefilter toward shorter wavelengths. Despite the desirability ofinterference filters, this band-pass dependance upon the angle ofincident light can seriously impair or degrade the precise functionrequired of an optical system.

For example, in correlation spectrometers of the type disclosedparticularly in U.S. Pat. No. 3,723,731 issued Mar. 27, 1973 to H.H.Blau, Jr., or in spectrometers of the type employing beam splitters asdisclosed in U.S. Pat. No. 3,488,491 issued Jan. 16, 1972 to M. Schuman,the systems may involve either a wide angle source of light as theprimary input or may involve a secondary source of light as acalibration source. In either event, the light from either the differentsources or from the various points on a single source will be incidentupon an interference filter in the optical train at different angles.The transmission by the filter will then vary according to the source orthe position of the points on a source, as the case may be. In aspectrophotometer which depends upon the simultaneous balance of lighttransmitted along separate absorbing and non-absorbing (or reference)paths to respective detectors, or which depends upon the balance betweenthe sequential transmissions of light through absorbing and referencepaths to a single detector, variations in the ratio of the detectedintensities, due to transmission changes in the input filter, willimpair the precision of the analysis performed by thespectrophotometers.

A principal object of the present invention is therefore to provide, fora filter having band-pass characteristics dependent upon the angle ofincidence of input light, an input optical system which renders thefilter relatively insensitive to the input angle of light to the opticalsystem.

Yet another important object of the present invention is to provide aspectrophotometeric device employing an interference filter which deviceis substantially insensitive to the angular characteristics of inputradiation to the device.

Other objects of the present invention will in part appear obvious andwill in part appear hereinafter. The invention accordingly comprises theapparatus possessing the construction, combination of elements, andarrangement of parts which are exemplified in the following detaileddisclosure, and the scope of the application of which will be indicatedin the claims. For a fuller understanding of the nature and objects ofthe present invention, reference should be had to the following detaileddescription taken in connection with the accompanying drawings wherein:

FIG. 1 is a perspective view showing the interior structure of anequalizer device of the present invention;

FIG. 2 is a perspective view, partly broken away, of another form ofequalizer device of the present invention;

FIG. 3 is a perspective view of a cylindrical element useful in yetanother form of equalizer of the present invention;

FIG. 4 is a cross-sectional view of still another form of equalizer ofthe present invention;

FIG. 5 is a schematic view of a correlation spectrometer employing theprinciples of the present invention;

FIG. 6 is a diagram partly in block form, illustrating the principles ofthe equalizer device of the present invention, and

FIG. 7 is a perspective view, partly broken away of an alternativeembodiment of an equalizer device similar to FIG. 1.

Generally the foregoing and other objects of the present invention areachieved by the provision of two optical devices or elements in sequenceas an input to an interference filter.

The first optical device, preferably termed an equalizer, provides thateach point or area element of its output aperture substantially exhibitsthe same emission intensity as every other point, regardless of theangular width of the source or even the number of sources providinginput radiation to the input aperture of the device. In other words,notwithstanding the nature of the distribution of input light to thedevice, the entire output aperture of the equalizer will be at oneuniform intensity which is substantially the average intensity of all ofthe input radiation. Positioned adjacent the output aperture of theequalizer is a collimator, the output aperture of the equalizer beingspaced from the collimator by the focal length of the latter. The outputof the collimator provides the input beam to the interference filter. Aswill be explained later, the total structure as thus described isrelatively insensitive to angular and spatial intensity variations ofthe input light to the system.

The spectrophotometer of the invention includes the foregoing threeelements, equalizer, collimator and filter as an input to a beamsplitter.

Generally, the equalizer of the present invention comprises an opticalsystem having an input aperture, an output aperture, and means forequalizing the distribution of light energy across the entire outputaperture.

The input and output apertures are not necessarily either the same sizeor shape, but for most practical situations they will be the same. In anideal equalizer, the power flow of radiation leaving any small region ofthe area of the output aperture would be the same as the power flow fromany other such small region of the output aperture. In a practicalequalizer, these regions need not be infinitesmal, and the shape of theoutput aperture may be restricted. For example, one could specify anequalizer having a circular output aperture with the regions of interestof the output aperture all being concentric rings of equal area. Thepower flow through each concentric annular region must then besubstantially the same as the power flow through any other such region.

An ideal equalizer should function regardless of the direction fromwhich light enters its input aperture, but in a practical equalizer thisis not necessary. An equalizer can be acceptable, even though there arecertain directions of incident light for which the equalizer does notwork, because it is possible simply to block light from entering theinput aperture from those directions without thereby causing anunacceptable diminution of the total light energy throughput.

In one embodiment, an equalizer of the present invention is formed of anelongated, light transmissive element having an input and outputaperture, the interior of the element being arranged so as to provide aprogressively greater number of discrete light transmitting pathsbetween the input and output aperture. In order to incur substantiallyequal light distribution at the termination of all of the paths at thecommon output aperture, at least part of each path is "leaky", i.e. canreceive and transmit light to one or more adjacent parts. Thus,typically the input aperture of the equalizer is divided into two lightpaths of substantially equal cross sectional area, having a commoninterface. For some predetermined distance, the interface is leaky,preferably by being approximately 50% transmissive and 50% reflective.For the remainder of the distance between the input and output aperture,the interface no longer permits leakage or cross-talk between thetransmissive paths on either side, and therefore is wholly reflective.Similarly, each of the original transmissive paths can be divided intosmaller and smaller transmissive channels each characteristically havinga leaky interface with an adjacent channel near the beginning of thetransmissive path and a light impervious (preferably reflective)interface toward the end of the path. All exterior surfaces of theequalizer are wholly reflective in order to minimize absorptiveattenuation and other power losses. One form of the device is shownparticularly in FIG. 1 and typically comprises an elongated volume 20formed of successive sections 22 and 24 of light-transmissive material.The term "light" as used herein is intended to include, whereappropriate, ultra-violet and infrared as well as visible radiation.Thus, for example the light-transmissive material of sections 22 and 24typically is glass or clear, high molecular-weight polymer for visibleradiation, alkali halides for infrared radiation and the like, and withdue consideration for the problems of supporting reflecting surfaces andof confining the transmissive material, can be one or more fluids. Ifreflective surfaces are self-supporting, the transmissive portions caneven be a vacuum, thus providing an equalizer usable in spectral regionswhere transparent materials are not readily available. Each of sections22 and 24 can be considered to be (and in fact can be constructed so asto possess) a plurality of flat, light-transmissive slabs havingsubstantially parallel planar interfaces.

Thus, for example, one can consider that there is a plane A--A extendingfrom a transparent end wall or input aperture 26 of section 22 so as todivide the latter into substantially equal halves or transmissionchannels to form a partially communicating interface between the twochannels. Lying in plane A--A and extending substantially transverselyto the general direction of light entering aperture 26, are a pluralityof spaced apart reflective elements or strips 28 and 30 (only two beingshown for the sake of convenience) so that for a distance d₁ extendinginwardly from aperture 26, substantially one half of plane A--A isreflective and the other half of plane A--A is transmissive. Strips 28and 30 can simply be formed of spaced-apart metallic foil ribbonsections which are clamped between a pair of light-transmissive slabs,or can be a deposited coating or the like.

Alternatively, a continuous film, typically of a dielectric halfreflective and half transmissive, can also be made. However, any otherarrangement, regular or random, of reflective portions and transmissiveportions (which themselves may be regular or irregular in shape andsize) is acceptable provided that such arrangement effectively isone-half transmissive and one-half reflective on the average. Startingat a distance d₁ from input aperture 26 and extending the remainder ofthe length of section 22 to output aperture 32, plane A--A includes anentirely reflective surface 31 so that substantially all radiationappearing at output aperture 32 is divided into two parts. At distanced₁ however, the two halves of section 22 provided by dividing plane A--Aare themselves each divided in half along planes B--B and C--C whichpreferably extend parallel to plane A--A, thereby subdividing theoriginal transmission channels each into a corresponding pair of smallertransmission channels.

For a distance d₂ from the end of distance d₁ , each of planes B--B andC--C is one-half reflective and one-half transmissive by the provisiontherein, for example, of spaced-apart reflective portions or strips suchas 34, 36, 38 and 40. Again, the number, size and arrangement ofreflective portions to be used in each plane is a matter of choice. Fromthe end of distance d₂ to output aperture 32, planes B--B and C--Cconstitute entirely reflective portions 39 and 41 respectively.

Thus, it will be seen that planes A--A, B--B and C--C have, within thedistance d₂, divided section 22 into four channels and consequently allof the light passing through section 22 into four distinct portions.Each of these latter channels is again divdied in half by respectiveplanes D--D, E--D, F--F and G--G which extend parallel to plane A--A andeach of which, for a distance d₃, is one-half reflective and one-halftransmissive by virtue of having a plurality of reflective strips suchas 42, 44 and the like which extend transversely across section 22 fromside to side of the latter. The process of dividing the channels inhalves can be continued indefinitely.

It will be apparent that by the structure disclosed, the light comingfrom output aperture 32 of section 22 will be divided into at least nparts where (n-1) is the number of dividing planes, n being a power of 2equal to or greater than 2. By virtue of the fact that for some initialdistance each of the planes is leaky because of being one-halfreflecting and one-half transmissive, light entrant into input aperture26 will tend to be evenly divided by each such plane. Thus it can beseen that to a fair approximation, the cross sectional area of outputaperture 32 will be divided into n area elements, each of which willhave distributed therein approximately a fraction of the total inputlight, i.e. 1/n. In order to maximize the transfer of the light frominput aperture 26 to output aperture 32 with optimum division, theexternal surface of section 22 with the exception of the input andoutput apertures, is preferably also covered with a reflective surface.It will be appreciated that the approximate division of the lightbecomes more and more nearly exact as n increases.

In some cases, equalization along a single direction, such as isprovided by section 22, is adequate. Where equalization along twodirections is required, section 22 may be followed by a similar section24. Directly coupled to output aperture 32 is second section 24 (only apart of which is shown) which typically is another unit identical tosection 22 in possessing a first plane H--H which is divided for adistance d₄ into one-half reflective and one-half transmissive surfaces,second planes J--J and K--K each of which corresponds to planes B--B andC--C of section 22 and so forth. Input aperture 50 of section 24 whichcorresponds to input aperture 26 of section 22, is directly coupled tooutput aperture 32, preferably so that no light leaks will occur, andwith plane H--H extending along a line common to but inclined at anangle greater than 0° and less than 180° (preferably 90°)to plane A--A.

It will be appreciated that radiation entering the equalizer nearlyparallel to the planes of section 22 will not be equalized. In mostcases, an acceptable solution to this problem is simply to prevent lightfrom entering too closely to a parallel with the planes of theequalizer, for example by turning the latter to an appropriate anglewith respect to the light source. In applications where it is criticalto minimize the range of angles of incidence of input radiation forwhich the equalizer does not work, the two sections 22 and 24 may befollowed by a third (not shown) having its planes disposed at 45° to theplanes of both sections 22 and 24. Alternatively, one can use twosections such as 22 coupled to one another, as shown in FIG. 7 such thatthe planes of one form a fixed dihedral angle with respect to the other,typically a shallow angle such as 170° or such. In order to insure afixed path length, joint section 51 in which all of the transmissionchannels such as 52 are separated from one another by fully reflectivesurfaces 53 which permit no light leakage between channels, is providedto couple the output of one section such as 22 to the input of the othersection such as 24.

Referring now to FIG. 2 there will be seen an alternative embodiment ofan equalizer of the present invention in the form of an elongatedstructure 52 formed of a plurality of stacked elements such as discs 54,55, 56 and the like. It should be understood that the discs, of whichdisc 54 will be described as exemplary, need not have circularcross-sections, and indeed the cross-section configuration is largely amatter of choice. Disc 54 however has two substantially planar facesonly one of which, face 58, is shown. Both of these planar faces areprovided with a light diffusing or scattering surface such as a groundor sand-blasted finish. Disc 54 is formed of a material transparent to adesired band of wavelengths and therefore can be formed of glassymaterials, crystals or the like. The external cylindrical surface ofstructure 52 is preferably covered or coated with a material 60 (shownbroken away) which is highly reflective to the wavelengths which thetransparent material of the discs will transmit. The planar faces ofadjacent discs are preferably in contact with one another.

Light entering one end of structure 52, as the face 58, is transmittedthrough the entire structure to the opposite end and is scattered ateach disc surface. Light leakage through the cylindrical periphery ofthe structure is minimized by the provision of the coating material 60.How evenly the structure of FIG. 2 distributes the intensity of lightacross its output aperture, as at end 62, depends upon the number ofscattering surfaces interposed between output aperture 62 and input face58. However inasmuch as a portion of the light entering through face 58will be back-scattered, the equalizer of FIG. 2 cannot operate at atransmission efficiency nearly as high as that of the equalizer of FIG.1.

In an alternative embodiment of FIG. 2, particularly useful inequalizing the distribution of an input beam of infrared radiation andwhich somewhat minimizes the inherent scattering losses, the discs areformed by pressing a mixed salt sample such as KBr and CsI. The saltsare initially in powder form and to achieve optimum mixing, the powderparticles should be in the 50 to 100μ range in diameter. A pellet ofmixed material will provide scattering because the two salts haveslightly different indices of refraction and the amount of scatteringachieved can be controlled by the proportion of the salts pressed into adisc. The matrix material can have either higher or lower index ofrefraction than the second material. Equalizers can be formed of aplurality of pellets, not necessarily all with the same scatteringproperties, either in direct contact with one another or separated fromone another by various appropriate distances by light conducting meanssuch as light pipes.

Alternatively discs or longer rods of scattering material can be madefrom mixed salt samples formed into a solid by heating under pressure ata temperature a few hundred degrees C below the melting point of thesalts and then cooling. By varying the time that the mixture is heldnear the melting point, controlled partial solution of one component inthe other will occur. This will produce a slowly varying change orgradiant of the index of refraction at the interfaces between thecomponents. By controlling the size of the scattering particles and theindex of refraction gradiant, one can manufacture a diffuser that willscatter predominantly in the forward direction into a relatively narrowcone of rays and hence provide high throughput. Mixtures of KBr and NaClare well suited for this purpose because of their similar melting pointsand indices of refraction; however, there are many choices of material.

In yet another alternative, the discs of FIG. 2 can be mixed so thatfully IR-transparent discs or lightpipes are sandwiched between discswhich scatter.

An alternative form of cylindrical element or disc such as 54 is usefulin a stacked array such as shown in FIG. 2 to form an equalizer is shownin FIG. 3. Disc 54 of FIG. 3 simply is a transparent element of aninfrared transmitting material such as pure KBr, having on at least oneplane surface 59 thereof, a refractive pattern such as a plurality oflenticules 60, much in the nature of a "fly's eye". Such discs diffuseby refracting rather than scattering and can form an efficientequalizer.

In any of the above-described infrared equalizers, it is desirable thatthe exterior surface be mirrored for IR reflection as by coating withspecular aluminum. In one particularly desirable form, coating material60 is a plural ply coating formed of an inner layer of aluminumprecoated on polyethylene terephthalate sheet, all enclosed in a heatshrunk polymeric tube. The technique for making such an equalizer simplyinvolves wrapping a selected stack of appropriate discs in thealuminized sheet, slipping the wrapped discs into the heat shrinkabletube and gently heating the latter sufficiently to shrink the tube downso as to tightly press the aluminum surface around the periphery of theequalizer while providing a mechanical bond which holds all of the discsin the equalizer in an appropriate position.

Controlled scattering throughout a volume can be achieved by use ofpowdered infrared transmitting material immersed in a transparent liquidhaving an index of refraction slightly different from that of the powderin the spectral region of interest. By controlling powder particle sizeand the difference in the indices of refraction, one can control thescattering properties of the mixture. One can for example, produce asystem having predominantly forward scattering into a relatively narrowcone and thus produce an equalizer with high throughput.

One embodiment of such an equalizer is shown in FIG. 4 wherein apowder-liquid system 62 is contained in cylindrical tube 63 with highlyreflecting inner walls 64. The ends of the tube are closed or sealedwith infrared windows 66. To be sure that there are no air bubbles,there is a side port 67 through which excess liquid can be introduced.

Alternately the equalizer of FIG. 4 can be formed of short sections ofpowder-liquid spaced with sections containing liquid only or transparentgas. In all cases the cell section containing powder is preferablypacked full so that there is no settling.

For the infrared, the liquids useful in system 62 can be CCl₄ and C₂ Cl₄with indices of refraction in the visible of 1.46 and 1.50. Powders foruse in system 62 can be BaF and KCl with indices in the visible of 1.47and 1.50 respectively. By mixing the two liquids one can get almost anydesired index match in that narrow range. There are obviously otherliquids and powders covering other ranges.

As shown in FIG. 5, the equalizer of the present invention findsparticular utility in a non-dispersive cross correlating spectrometricsystem such as the correlation spectrometers of the type described inthe patents to Blau, Jr. and Schuman earlier identified. Thus, thespectrometer of FIG. 1, basically employs a scheme described in detailin the Blau, Jr. U.S. Pat. No. 3,723,731 in having an optical filter 70,preferably an interference filter, which provides a very narrow bandpass, so as to be capable of isolating a band of frequencies within theabsorption band of gas to be detected due to radiation passing throughsample region 72. Light which has passed through filter 72 is thenfocused by lens system 74 and is caused to traverse alternately and inrapid sequence each of a plurality of cells shown as an array 76. Lightwhich has passed through a respective cell of array 76 is split by beamsplitter 68 into two beams. A first of the beams is directed by lens 80to radiant energy detector 82 such as an infrared-sensitive bolometer.The other of the beams is transmitted through reference cell 84 whichcontains a gas mixture with the same total and partial pressures as agas mixture in one of the cells of array 76. Light transmitted throughcell 84 is then focused by lens 86 on to radiant energy detector 88which serves, in the same manner as does detector 82, to convertincident radiant energy into electrical signals.

It will be seen that in the device of FIG. 5, equalizer 90, which cantake any of the forms heretofore described, is provided in the path ofradiation which has traversed sample region 72. Output aperture 92 ofequalizer 90 is disposed at the focus of a collimating system such aslens 94, lens 94 being disposed so that its collimated output isincident substantially perpendicularly to the mean plane of interferencefilter 70. If however, the equalizer used is a divider-type such as isshown in FIG. 1 it is desirable, to block the center of transfer lens 74so that rays traversing equalizer 90 "straight-through" are not used.

The system of FIG. 5 will operate substantially as described inaforesaid U.S. Pat. No. 3,723,731 insofar as the sequence of parts fromfilter 70 through array 76 to detectors 80 and 88 is concerned. However,the provision of equalizer 90 and collimater 94 in sequence as elementspreceeding the interference filter 70, confers upon the system asreduced sensitivity to variation in the distribution of input light fromsample region 72. For such purposes, other equalizers can also be usedif perhaps not as satisfactorily as the equalizers heretofore described.For example, one can employ a kaleidoscope provided that the "straightthrough" rays are also blocked, or a torsional light conductor asdescribed in U.S. Pat. No. 3,752,561, or bent fiber optics or the like.

This insensitivity of the system to variations in angular input andintensity can be explained with reference to FIG. 6. Two exemplary lightsources are shown as S₁ and s.sub. 2 having different radiantintensities, and each providing input light beams 96 and 98 withdifferent respective angular orientations. Light beams 96 and 98 enterequalizer 90 which, as heretofore explained, has an exit aperture area,each area element of which provides substantially the same lightintensity as each other area element thereof, regardless of thedistribution characteristics of the input beam of the device. Two sucharea elements are identified as E₁ and E₂, the former of which lies onthe optical axis M--M of collimating lens 94, the latter being locatedoff-axis. Thus, for source S₁ by definition the light intensities I_(E1)and I_(E2) provided respectively at E₁ and E₂. are equal. Inasmuch as E₁lies on the optical axis of lens 94 light from E₁ when collimated bylens 94 is projected in rays (shown as dotted lines) parallel to axisM--M of the lens. Lens 94 also collimated light from E₂ and projects itin rays (shown as dashed lines) forming some angle φ with optical axisM--M. But because I_(E1) El = I_(E2) the amount of energy passingthrough filter at φ is the same as when φ = 0. These considerations areexactly the same for source S₂ (except of course that the energy I atany area element at the output of equalizer may have different valuesfor S₁ than for S₂). One can then state that the total energy emittedacross the output aperture of equalizer 90 has precisely the sameconfiguration regardless of the angular distribution and the intensityvalue of the input light to the equalizer. Because the filter then seesthe same energy distribution although the energy amplitude may bedifferent, the filter passband remains invariant with respect to theinput angles and intensity distribution of the input light to thesystem. The system will still work even if the two intensities are notequal provided that they have the same relationship to both sources.However, for equalizers that produce an output intensity distributionthat is independent of the distribution of the incident light, thatoutput intensity distribution is ordinarily uniform.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawing shall be interpreted in an illustrative and notin a limiting sense.

What is claimed is:
 1. A light equalizer for providing substantiallyuniform distribution of light within a spectral region of interestacross an output aperture, said equalizer havingan enclosure havingopposite ends transparent to said light, a body of fluid disposed insaid enclosure and being transparent to said light, a plurality ofminute particles distributed in said fluid and being transparent to saidlight, said particles and said fluid having different indices ofrefraction with respect to said light within said spectral region; andinternally wholly reflective means disposed about the entire exterior ofsaid enclosure except for said opposite ends.